std::fstream myFile("/home/adam/Documents/cryptopp/mackeystring.dat",std::ios::app);
 
myFile.open("/home/adam/Documents/cryptopp/mackeystring.dat");
 
if (! myFile) // Always test file open
    {
        std::cout << "Error opening output file\n";
        return -1;
    }
 
//char *key;
 
	std::string keystring;
 
	getline(myFile,keystring);
 
unsigned char sequence [1];
memcpy(sequence,&keystring,1);
 
myFile.close();
 
memset( key, sequence ,20);

unsigned char key[20];
fstream f(".....",ios::binary);
// No need to open file explicitly 
// when the filename is specified in the constructor.
if (f.is_open()) // or simply: if (f)
{
    if (f.read((char*)key,sizeof key))
    {
    // you get this data!
    }
}

std::string keystring;
...
unsigned char sequence [1];
memcpy(sequence,&keystring,1);

memcpy(sequence,keystring.data(),1);

if (myFile.read((char*)key,20))
{
    // Well, you gOt your key! It's here, in key unsigned char array now!
    // (char*)key - it's a proper cast here!
    .... don't worry, process your key as unsigned char if you wish...
 

unsigned char key[20];
 
fstream myFile ("/home/adam/Documents/cryptopp/mackeystring.dat", ios::binary);
myFile.open("/home/adam/Documents/cryptopp/mackeystring.dat");
 
if (myFile)
{
 
myFile.read((char*)key,sizeof key);
}
 
else printf("error opening file");
 
myFile.close();
 
printf("\nFILE CONTENTS: \n");
 
for (int i=0; i<20; i++)
printf(" %02x ", key[i]);
 
//full content of key= 30  62  30  62  30  62  30  62  30  62  30  62  30  62  30  62  30  62  30  62 
 
//this is where the 20 byte key is used:
HMAC<SHA1> hmac( key, sizeof(key ) );
...
...